Recently, a thin film produced by a sputtering process is widely used in electronic devices such as a semiconductor device, a liquid crystal display, and a magnetic recording device. In the sputtering process, a base material referred to as a target material and a substrate are positioned to face each other in a vacuum vessel, and glow discharge is generated on a surface of the target material while introducing an inert gas, such as Ar, into the vacuum vessel, whereby forming a thin film of an element on the substrate, which element forms the target material.
A target material used as a base material in a sputtering process is required to have a uniform structure and a reduced content of impurities. Oxygen, amongst impurities, is caught in the thin film whereby deteriorating properties thereof. If oxygen is present as an oxide included in a structure of the target material, it is considered that abnormal electric discharge occurs during sputtering, so that an oxygen decrease is strongly desired.
A process of producing a target material is generally classified to be a melting method and a powder sintering method. However, in the case of a target material made of a metal element having a high melting point, it is hard to melt the target material, and also to subject the target material to plastic working in order to homogenize the material structure, so that such a target material has been often produced by the powder sintering method. However, the powder sintering method involves a defect that since powder particles of a powder used in the method have a large specific surface area, a relative amount of oxidized layers formed on the surfaces of the powder particle is high, the target material produced by the powder sintering method is liable to contain a higher amount of oxygen than that produced by the melting method. Especially, in the case where the powder particles have a porous structure, a sponge like structure or a dendritic structure each having a large specific surface area, the above defect is liable to be outstanding.
Accordingly, in general there has been adopted an oxygen decreasing method according to which a powder is subjected to heat treatment in an atmosphere, in which a reducing gas, such as hydrogen, is introduced, whereby reducing the oxidized layers on the powder particles.
Alternatively, the present applicant proposed a new method of decreasing oxygen content in a refractory metal powder, according to which method the refractory metal powder is introduced into thermal plasma flame, in which a hydrogen gas is introduced, whereby refining (i.e. deoxidizing) the refractory metal powder (see JP-A-2001-20065, for example).